Can anyone explain why?
If you think about it, without having the data of the heat of combustion of each type of bond, you can't really answer this question. I know that cyclopropane has a higher heat of combustion PER BOND than cyclobutane. However, that, in my sense of viewing the problem, does not prevent the fact ethylcyclobutane could have a higher heat of combustion as it is a cycloalkane with 4 bonds rather than 3.
It could happen that the three bonds in cyclopropane and the three bonds in the alkyl branch are lower in energy that the four bonds in cyclobutane and the two bonds in the alkyl branch.
For example: Let's say a cyclopropane C-C bond is 10J. A cyclobutane C-C bond is 9.5J. A regular alkane bond is 8J.
For propylcyclopropane: Total energy=54J For ethylcyclobutane: Total energy=54J The same! I know this sounds really simplistic and dumb, but I just don't understand how you can assume the answer is A.
How do you analyze this situation? Please help.